The present invention relates, in general, to evaluating the operation of imaging equipment and, more particularly, to charts and methods for evaluating the image quality of such equipment by approximating a modulation transfer function. While the present invention is generally applicable to a variety of imaging equipment it will be described with reference to an optical instrument, such as a laparoscope, for which it is particularly applicable and is being initially applied.
The quality of optical/video/imaging systems is often characterized in terms of a modulation transfer function (MTF). A typical MTF measurement system uses a target defining a pattern having a series of alternating light and dark parallel lines, which are imaged by the system under test. A "line" is defined as consisting of one light bar and one dark bar, i.e., one light image cycle.
The target pattern has several series of parallel lines with each series of lines having different spacing, expressed as a line frequency or number of lines per millimeter. The series may range from a coarse line, for example 1 line per millimeter, to a fine line, for example 10 lines per millimeter.
To evaluate an optical instrument, the target pattern is imaged through the optical instrument and the amount of light passing through the instrument is detected as the instrument is scanned from the coarse end of the pattern toward its fine end. The amount of light detected varies as the scan progresses, and an image modulation is given by: ##EQU1## where I.sub.max and I.sub.min represent the maximum and minimum image illumination on the detector. The object modulation, M.sub.o, is similarly derived from the maximum and minimum brightness levels of the object: ##EQU2## The modulation transfer function is then defined as the ratio of the image modulation to the object modulation, M.sub.i : M.sub.o.
Unfortunately, the measurements required previously to derive a modulation transfer function are costly and timeconsuming. For example, the measurements must be performed in a laboratory having complicated equipment requiring skilled technicians for maintenance and operation of the equipment.
It is, thus, apparent, that there is a need for a simplified technique for quickly and inexpensively evaluating the image quality of an optical instrument. Preferably, such a technique would allow the operator of an optical instrument to directly evaluate the performance of an optical instrument at the time the instrument is to be used.